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Death row inmate executed using pentobarbital in lethal injection

John David Duty was sentenced to die for killing his cellmate while he was incarcerated on rape and robbery charges.

STORY HIGHLIGHTS

NEW: John David Duty pronounced dead at 6:18 p.m. CT

Pentobartibal is used to euthanize animals

Sodium thiopental, usually used for executions, is in short supply

(CNN) -- An Oklahoma death row inmate received a drug commonly used to euthanize animals Thursday because of a nationwide shortage of sodium thiopental, the drug usually used as the sedative in its three-drug execution cocktail.

John David Duty was convicted and sentenced to die for strangling his 22-year-old cellmate, Curtis Wise, with shoe laces in 2001. At the time, he was serving three life sentences for rape, robbery and shooting with intent to kill from a 1978 conviction.

"To the family of Curtis Wise, I'd like to make my apology. I hope one day you will be able to forgive me, not for my sake but for your own. My family and friends are here too. Thank you. You've all been a blessing. Thank you. Lord Jesus, I am ready to go home," Duty said before he was executed.

Duty's execution was the last in the United States in 2010 and is believed to be the first in the country to use pentobarbital in a lethal injection.

Sodium thiopental is a rapid-onset, short-acting barbiturate that causes unconsciousness. Duty's attorneys argued that pentobarbital was risky and unsafe. But an Oklahoma judge disagreed and last month approved its use in place of sodium thiopental.

The sedative is the first drug in Oklahoma's lethal injection protocol. It is followed by vecuronium bromide, a drug that causes paralysis and stops breathing. The third drug, potassium chloride, stops the heart.